The emerging field of printed electronics promises to create a wide range of consumable electronic devices, many of which will be manufactured at very high volumes and very low cost. It is the ability to directly print these devices using known printing techniques that promises to keep their cost of production low by minimizing energy, materials consumption and environmental impact. For instance, while semiconductor transistor devices have historically been manufactured on silicon wafers under high vacuum conditions in expensive cleanroom type environments, transistor devices can now be directly printed onto inexpensive substrates using common bench top printing devices, such as ink jet printers. Additionally, the use of a printing press permits the ‘direct write’ of materials, such that a multilayered device can be constructed without wasting materials, which keeps costs down and lowers the environmental burden associated with disposing of waste materials.
By contrast, silicon wafer type electronic devices are built up in a multilayered process, whereby layers, such as dielectrics, are blanket deposited over the entire wafer, photolithograpically patterned and then etched leaving the desired pattern intact. Similarly, metal layers of copper are deposited on the wafers, and then, excess copper is removed in a chemical mechanical polishing step.
Both of these processes are thus ‘subtractive’ in that a material layer is first applied and then portions of it removed to leave behind the desired pattern or shape. Thus, excess material that is consumed in the first step becomes waste in the second step. This is especially problematic for metals such as copper since it constitutes a toxic waste.
Besides providing an alternative low cost manufacturing pathway for electronic devices, there are also new emerging technologies that promise to be enabled by the innovation of printed electronics. An example of this is the rapidly expanding field of Radio Frequency Identification (RFID) tags. This revolutionary technology comprises of a microchip connected to a metallic antenna, which when subjected to a specific radio frequency from a ‘reader’ unit, activates the chip to broadcast its identity profile back through the antenna to the reader. In this way inventory, for instance, can be instantaneously read by driving by with a reader unit, or items can be automatically registered at a checkout counter. At higher levels of functionality, these devices can be integrated into wireless networks to form the backbone of a ‘smart environment’ where, for instance, the temperature, lighting and entertainment of a home can be seamlessly adjusted by sensing the homeowners presence and movement by RFID. In all cases, a key requirement is that the RFID tag has a highly conducting metallic antenna and, due to the severe pricing pressure associated with the tags, it is widely accepted that this feature will need to be directly printed using metallic inks, preferably copper. Other applications where the direct printing of metals, such as copper, would be particularly beneficial include the direct write of circuit boards for the mounting and interconnection of electronic systems. These boards comprise a grid of conducting copper lines mounted on an insulating substrate. The current process for manufacturing them essentially begins with a sheet of copper which is patterned with the desired copper grid layout, and then the copper sheet is etched to leave only the pattern remaining. Thus, >90% of the copper is eliminated as a waste, which must be disposed of.
Other examples of electrical devices, which need to be fabricated using printing techniques are display screens, which can be flexible. Typically, these screens need an electrically conducting matrix to be printed on one surface to address and illuminate individual pixels, which provide an image, or they need conductive pathways printed to power light emitting display sources, such as organic light emitting diodes (“OLEDs”).
There are many commercially available metal inks for the above and other applications, including silver inks and copper inks, in addition to those for other metals, such as nickel. In general, it is highly advantageous to create metal inks, which can be processed at low temperatures, since this permits their printing and curing to be performed on thermally sensitive substrates, such as polymers and papers, which are prized for their ready availability, flexibility and low cost. In general, inks which can be cured below 150° C. are the most desirable.
There are many printing techniques amenable to the manufacture of printed electronics including, but not limited to, gravure, silk screen, litho and ink jet. However, the most rapidly evolving technique amongst these is ink jet, due to its ability to be programmed to print a wide variety of different patterns and overlays compared to the other techniques, which rely on the printing from an essentially fixed pattern embossed or etched in some way in the form of a master die, which is repetitively used. In addition, there are constant improvements in ink jet technology, which are driving the ink drop sizes smaller, their placement on the substrate more accurate and increasing the number of printing nozzles on a print head, currently available at approximately 4000 nozzles per head, to provide high density printing for high throughput, including reel-to reel capability.
Silver, being the most conductive metal with a resistivity of 1.6 microohmcm, currently enjoys the most widespread use as an ink, which can be printed to give conducting films of metal. There exists a wide range of silver inks of varying viscosities and chemical formulation, which can be selected from depending upon the exact application and printing technique to be used. For instance, if silk screen printing is used, then a relatively high viscosity ink >1000 Cps can be utilized.
By comparison, a low viscosity silver ink <100 Cps, such as used by Cabot, would be selected if ink jet printing was to be used. Typically, these silver inks are comprised mostly of silver metal particles ranging from multi-micron or sub-micron particles, that are approximately spherical or flake-like in shape to ‘nano-powder’ inks of <200 nm diameter metal particles. Metal powder loading of these inks can range from 20-80 wt % and are typically mixed with a liquid medium, which is selected to be an agent to enhance the printability of the ink and/or to participate chemically in the thermal rendering of the ink into a conducting silver line. For instance, the chemical agent added to the silver particles may participate in the sintering of the silver particles together into an electrically connecting and conducting mass. Additionally, if a silver containing compound is present in the silver ink (either as a deliberately added specie or specie formed in situ), which can thermally break down to form fresh silver metal, then this fresh metal will aid in the fusion of silver particles to each other to form a conductive matrix. Many silver inks can be thermally cured below 250° C., some below 200° C., and some below 150° C. The lower temperature inks are the most desired for many printing applications. Since silver is a relatively noble metal, whose oxide is electrically conductive, silver inks can be printed and thermally cured in air, without deleteriously affecting their electrical performance. Silver's electrically conducting oxide also means, that when particles of silver are in intimate contact, they will establish electrical interconnectivity, even if they are coated with it. However, not withstanding the high electrical conductivity and relative ease with which conductive inks can be made from silver, it remains a relatively expensive metal, prohibitively so, for very high volume and low cost printed electronics devices. Copper, with a resistivity of 1.67 microohmcm, is almost as conductive as silver, but has a greater capacity to handle high current loads due to its superior electromigration resistance. Additionally, copper is more abundant and less expensive than silver, which makes it a very attractive candidate for conducting inks. There are however, challenges associated with creating copper inks, which can yield conducting lines of metal at temperatures <150° C.
Unlike silver, the oxide coatings on copper particles are found to be insulating rather than conducting. Therefore, if an ink comprising mostly copper metal powder is prepared, a chemical technique must be established whereby the oxide can be effectively removed during the thermal cure step to permit the particles to come into intimate contact and thereby establish an electrically conducting matrix. Additionally, a specie or species may be either introduced, or created in situ, which thermally breaks down to yield fresh copper metal, which also aids in fusing the copper particles together. Therefore, if a copper metal powder is brought into contact with an acidic species capable of removing the oxide, as the mixture is heated to a cure temperature the resultant copper compound, which forms from the oxide, breaks down to give fresh copper metal. Copper metal particle sintering is then expected to occur. In other words, the copper particles are cleaned of oxide, and then, effectively welded together. This represents the technology of Kydd (U.S. Pat. No. 6,036,889), where copper powder is mixed with neodecanoic acid, which upon heating this mixture to >250° C. results in a conducive copper trace. However, curing below this temperature results in the above mechanism not being fully operative, resulting in an incompletely sintered and poorly conducting metal film of poor mechanical properties. Therefore, these inks can not be used to print and cure copper lines onto thermally sensitive substrates, where a cure below 150° C. is desired. In a similar approach, Kydd also describes (U.S. Pat. No. 6,824,603) using mixed metal oxide powders with acid type components which react together to give a metal compound, which in turn breaks down to give metal during a thermal cure. However, this too is distinct and different from the present invention.
Also unlike silver, copper inks can not be effectively thermally processed in air, since the resultant copper film will undergo oxidation to copper oxides, which will raise the resistivity of the resulting copper film. Therefore, it is desirable to thermally cure copper inks in an inert atmosphere. For copper inks, silver inks and other metal inks, having the inks comprised mostly of metal powder represents an approach by which to maximize the metal ‘loading’ of an ink. However, for some applications, such as ink jet printing, where the ink needs to be ejected through a very small diameter orifice, typically <100 microns, metal powders can lead to problems with clogging the jets and settling out by sedimentation upon standing.